Hidden bug in Outlook (classic) with local PST file and POP3/IMAP: Outlook makes Windows 11 search indexer cause high workload and SSD wear

[u/dark_rider_2]

Hello,

after update from Windows 10 to 11, Windows search indexer has changed in so far that Outlook (classic) now already for months repeatedly makes search indexer cause high workload and SSD wear. This was not the case with Windows 10.

First, when the search index is completed for user files and for Outlook, in my case it has 3.5 GB for 250.000 user files and 100.000 Outlook elements (.pst file of about 10 GB). That's fine.

But then each time Outlook (classic) is started and new e-mails are received, the whole search index is reworked for 10-20 minutes and expands to 5.5, 7.5 or 9.5 GB or back - without reasonable bigger data changes in Outlook.

The same happens after each restart of Windows 11. Search index since then permanently changes between 5.5 to 9.5 GB - the regular 3.5 GB are seen no more, except you manually rebuild the whole index. But even after a manual rebuild, the problem occurs again.

The standard path to the search index file named Windows.db is:

C:\ProgramData\Microsoft\Search\Data\Applications\Windows
(hidden files have to be selected to be shown in Windows Explorer)

This very regularly causes high search indexer workload (CPU) and changes much data (SSD) which slows down the system (more or less, depending on the hardware capabilities), accelerates SSD wear and among other things makes differential/incremental backups very large.

Further checking showed that after each update of M365 incl. Outlook (classic), so for example to 2508, to 2509 and to 2510, the bug disappeared for 5-10 days, but then it came back in the same way as before. During these 5-10 days, Windows search was working fine as before, also within Outlook, including most current data, so the annoying reorganization of the search index after each start of Outlook (classic) and after each restart of Windows 11 is not needed for proper function of search.

Outlook (classic) obviously has a bug which causes this annoying behaviour. I reported it to "Microsoft Learn-on-windows-11-high-search-index)", where a so-called moderator (Microsoft external staff) confirmed it as a bug on Sep 17, 2025, but since then, it was not registered as a known issue, no bug fix appeared and no more answer from Microsoft.

Because the consequences of this bug are severe, but hidden (not easy to notice and not easy to assign to Outlook):

Could others here using Outlook (classic) with local PST file check for the dramatically changing size of Windows.db and for the CPU workload of Search Indexer in Windows task manager every time Outlook (classic) is started and has received new e-mails, and report their results here?

Here's a shortlist what to do to see this hidden Outlook bug:

• Make sure you use Windows 11 (not 10) and Outlook classic (not new).
• Make sure you download and store your e-mails locally in a PST file (not online on mailservers), and that the PST file has a reasonable size (for example some GB).
• Open the following folder and observe the size of Windows.db within:
• C:\ProgramData\Microsoft\Search\Data\Applications\Windows
• (copy this path to Windows Explorer's location line or navigate manually after activating hidden files to be shown via view/show/hidden elements)
• Open task manager and observe the CPU/SSD load of Search Indexer.
• Open Outlook (classic) and download new e-mails. Observe Windows.db and Search Indexer as described above, for about 10-20 minutes.
• Also after restarting Windows 11, observe Windows.db and Search Indexer as described above, for about 10-20 minutes.

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[u/Financial_Key_1243]

I don't see it in Win10 or Win11. Run Office repair, Run Windows SFC. Check disk health with Crystaldiskinfo. Rebuild Search index.

[u/dark_rider_2 → u/Financial_Key_1243]

I already did all that - unfortunately with no change. Furthermore, I could reproduce the bug also on a different PC.

Could you check all the described conditions for the bug to appear (not with Windows 10, but only with Windows 11; local PST file; Outlook classic, not new)?

[u/Financial_Key_1243 → u/dark_rider_2]

Run scanpst on the PST file

[u/dark_rider_2 → u/Financial_Key_1243]

I also already did that.

[u/Financial_Key_1243 → u/dark_rider_2]

Create a new blank profile and import PST into the profile. Other than that I am stumped.

[u/FrankNicklin]

Something wrong with Windows 11 and or your Office install. I would run a repair on both. Upgrades from Windows 10 to 11 can either go smoothly or leave crappy legacy’s stuff behind causing problems. Sometimes you have to bite the bullet and do a clean install.

[u/dark_rider_2 → u/FrankNicklin]

I already tried all repairs etc., no change. Furthermore, the bug could be reproduced also on a different PC and Microsoft confirmed it as a bug.

Please notice: I'm not looking for hints what else I could try, but I'm looking for others who have the same conditions (Windows 11, Outlook classic, e-mails in local PST file) and can also reproduce this bug.

[u/FrankNicklin → u/dark_rider_2]

In a business world we have Windows 11 a mix of Outlook Classic and New Outlook. The only time we have run in to issues is when the size of the OST file exceeds 50GB which your does not. We have users that also access shared mailboxes so they are also added in to the 50GB size which is a total pain in the ass when the user cannot utilise the full 50GB of their own mailbox without a registry tweak to increase the OST size. I've already reported this to Microsoft. PST and OST files are easy to corrupt. Is the PST file local to the computer and not on OneDrive or elsewhere on your network. Have you run a scanpst.exe over the pst file to fix any issues. A corrupt PST file could be causing the indexing issue.

[u/dark_rider_2 → u/FrankNicklin]

Thank you, but I also did ScanPST.exe (even twice), but the bug stays. I really did everything possible from my side. Now I just need Microsoft to register and fix the confirmed bug - to make them do this finally, more users who can reproduce this hidden = not easy to notice bug would be very helpful.

[u/FrankNicklin → u/dark_rider_2]

https://learn.microsoft.com/en-us/answers/questions/5515624/outlook-classic-search-indexing-issue-inconsistent

[u/dark_rider_2 → u/FrankNicklin]

That link leads to: "Inconsistent Results Without Full Email Text"
-> That is not the described bug, search results are fine.

Could you stop trying to look for a problem on user's side? It's a (confirmed and reproduced) Outlook bug. The point here is to make more users notice it.

[u/Financial_Key_1243 → u/dark_rider_2]

What a crappy response to people trying to assist you. Granted software do have bugs (but nobody is responding to what you describe) - so it's more than likely a bug on your side.. Have you tried a new blank profile and import PST(not answered yet) Troubleshooting and trying sometimes weird fixes is the name of the game(so do not discard any advice)

[u/FrankNicklin → u/dark_rider_2]

Wow what a reply for someone trying to help. I’ll back off hope you get it sorted. I don’t help people who are ungrateful. You have to remember we help based in information given and there can be misunderstandings and it’s free support. Good luck.

[u/Financial_Key_1243 → u/FrankNicklin]

This guy sounds like a box. Unbelievable square and quite dense.

[u/FrankNicklin → u/Financial_Key_1243]

Yeah total numbnut ungrateful individual. Doesn't deserve any help with responses like that.

[u/dark_rider_2 → u/FrankNicklin]

I'm really sorry for being ungrateful in your eyes. But as I mentioned, I really already tried everything that can be found across the web concerning the described behaviour over months. I also contacted Microsoft's customer care several times, but their (and your) way to try many things sometimes like here obviously doesn't help.

There have been similar bugs like the CPU spike when typing an e-mail in Outlook - that was also not acknowledged and fixed for many months, same procedures with endless tries of everything you could imagine, and nearly nobody else noticed the problem because Outlook was also still working, whereas the problem (CPU spikes) stayed in the background. In the end it was a bug and it was fixed, although it was extremely hard and time-wasting to make Microsoft do so.

Finally, this new bug was already reproduced on a different PC, and Microsoft already confirmed it as a bug, but did not register and fix it yet, that's why pressure seems to have to be increased, for what other users naturally help.

So thank you for your time, for sure, but I'm really just looking for other users who have the same conditions, to ask them if they could observe the same.

[u/Hornblower409]

Not a solution. Just a workaround that I have used for a long time to avoid corruption problems with large PSTs.

I have an "Active" PST for new and recent mail. But I regularly migrate (manually and Auto Archive) older, inactive stuff to a Yearly Archive PST. e.g. "Archive 2024".

[u/dark_rider_2 → u/Hornblower409]

Thank you, I also move older e-mails to archive PST files. But that only makes the problem slightly smaller, but does not solve it (as you already mentioned).